Curable compositions containing polymerizable monomers and radical polymerization initiators are widely used in various applications such as coating materials, printing materials, adhesive materials, modeling materials, sealing materials, and dental materials. Among these, in dental applications, curable compositions containing, as polymerizable monomers, (meth)acrylate polymerizable monomers or (meth)acrylamide polymerizable monomers are in practical use as dental materials such as dental cements, dental adhesives, dental composite resins, dental autopolymerizing resins, etc.
A type of common radical polymerization initiator is a polymerization initiator composed of a combination of an oxidizing agent and a reducing agent. When an oxidizing agent and a reducing agent are mixed, a so-called redox reaction occurs to generate radicals, so that the generated radicals initiate a polymerization reaction to promote curing of a composition.
A specific example of the combination is, for example, a combination of a benzolyl peroxide as an oxidizing agent and an amine compound as a reducing agent. However, benzoyl peroxides have the disadvantage of being hard to handle because, due to their low thermal stability and poor storage stability, they must be kept refrigerated. On the other hand, amine compounds are easily converted into colored substances by chemical changes. Due to their susceptibility to coloring, amine compounds have the disadvantage of being hard to use in applications that require aesthetic value such as dental applications.
To address this problem, Patent Literatures 1 and 2 propose, as an improved radical polymerization initiator free of these disadvantages, a combination of a hydroperoxide and an acyclic thiourea compound. Patent Literature 3 proposes, as a radical polymerization initiator, a combination of a hydroperoxide and a cyclic thiourea compound having an unsubstituted cyclic structure. Hydroperoxides are more thermally stable than benzoyl peroxides or the like and are advantageous in storage stability. Unlike amine compounds, thiourea compounds are resistant to coloring.